IR Thermography Primer

Answers to Common Questions People Ask about IR Thermography

What is infrared?Infrared energy is part of the electromagnetic spectrum and behaves similarly to visible light. It travels through space at the speed of light and can be reflected, refracted, absorbed, and emitted. The wavelength of IR energy is about an order of magnitude longer than visible light, between 0.7 and 1000 µm (millionths of a meter). Other common forms of electromagnetic radiation include radio, ultraviolet, and x-ray. More information

The electromagnetic spectrum.

What is the electromagnetic spectrum?We know that infrared radiation is a form of electromagnetic radiation, which is longer in wavelength than visible light. Other types of electromagnetic radiation include x-rays, ultraviolet rays, radio waves, etc. Electromagnetic radiation is categorized by wavelength or frequency. Broadcast radio stations are identified by their frequency, usually in kilohertz (kHz) or megahertz (MHz). Infrared detectors or systems are categorized by their wavelength. The unit of measurement used is the micrometer, or micron, (mm, where m is the Greek letter mu) which is one millionth of a meter. A system that can detect radiation in the 8 to 12 mm band we usually call "longwave." One that detects radiation between 3 to 5 mm is termed "shortwave." (A 3 to 5 mm system can also be classified as "midband," because there are systems which can detect radiation shorter than 3 mm.) The visible part of the electromagnetic spectrum falls between 0.4 and 0.75 mm. We can see colors because we can discriminate between different wavelengths. If you have a laser pointer you may have noticed that the radiation is specified in nanometers; usually about 650nm. If you examine a chart of the electromagnetic spectrum at 650nm (.65 mm) you will see that it is the radiation of red light.

Where does infrared energy come from?All objects emit infrared radiation as a function of their temperature. This means all objects emit infrared radiation. Infrared energy is generated by the vibration and rotation of atoms and molecules. The higher the temperature of an object, the more the motion and hence the more infrared energy is emitted. This is the energy detected by infrared cameras. The cameras do not see temperatures, they detect thermal radiation.

At absolute zero (-273.16°C, -459.67°F), material is at its lowest energy state so infrared radiation is at its lowest level.

What is infrared thermography?Infrared Thermography is the technique for producing an image of invisible (to our eyes) infrared light emitted by objects due to their thermal condition. The most typical type of thermography camera resembles a typical camcorder and produces a live TV picture of heat radiation. More sophisticated cameras can actually measure the temperatures of any object or surface in the image and produce false color images that make interpretation of thermal patterns easier. An image produced by an infrared camera is called a thermogram or sometimes a thermograph.

Black and white and color thermograms of a person, and a visible light photograph. Note the glasses appear cool because they are cooler than the skin and longwave infrared energy will not pass through glass. You can see the temperature patterns on the face: reds are warmer, yellows and greens are cooler. Thermal patterns on the skin surface can be indicative of disease and are sometimes used to aid medical diagnoses.

How is thermal imaging different from "night vision" goggles?Night vision goggles amplify small amounts of visible light (and sometimes near infrared light) thousands of times so objects can be seen at night. These only work if some light is present, e.g., moonlight or starlight. Thermal imaging works by detecting the heat energy being radiated by objects and requires absolutely no light. One advantage of thermography over night vision technologies is that night vision goggles can be easily blinded just by shining a flashlight at them. Since thermal imagers only look at the heat, they are totally unaffected by light sources.

Visible light

Near infrared "night vision"

Thermal infrared

I have seen movies where thermal imaging is used to "see through" walls. Can this really be done?Unfortunately this is pure Hollywood fiction. However, there are at least a couple of films with real thermal infrared footage: Predator and Predator 2. The authors of this article were consultants on these two films. These were made using a single detector scanning system with a liquid nitrogen cooled detector. Today we use room temperature focal plane arrays.

Can thermal imaging be used for hunting?Thermal imagers work great for detecting and finding people and animals in complete darkness; however, they are illegal for hunting purposes.

If IR cameras don't see temperature, what am I seeing on an IR image?The IR camera captures the radiosity of the target it is viewing. Radiosity is defined as the infrared energy coming from a target modulated by the intervening atmosphere, and consists of emitted, reflected, and sometimes transmitted IR energy. An opaque target has a transmittance of zero. The colors on an IR image vary due to variations in radiosity. The radiosity of an opaque target can vary due to the target temperature, target emissivity, and reflected radiant energy variations.

The accompanying figure shows three metal cans: one hot, one ambient, and one cold (left to right). The upper image is visual while the lower image is infrared. There is a piece of electrical tape on each can. The can surface and the electrical tape are at the same temperature for each can. But in the infrared images, the tape looks hotter than the metal surface on the hot can, colder on the cold can, and the same on the ambient can. What is going on?

The electrical tape has a higher emissivity than the metal. This means the tape has a higher efficiency as a radiator than metal. The metal has a higher reflectivity than the tape. It is more efficient as an infrared mirror. Thus, the tape will indicate the target temperature more closely. The metal will indicate the background temperature, or that which is reflected off the can. So, if the can is hotter than the background, the tape looks hotter than the metal. If the can is colder than the background, the tape looks colder than the metal. If the can is the same temperature as the background, the tape and the metal will look the same.

Three aluminum cans with partially oxidized surfaces and a strip of black electrical tape. The can on the left is hot, the middle can is ambient and the right can is cold.

This is an extremely important concept. Thermographers see targets exhibiting this emissivity contrast behavior every day. It could be an insulated electric cable with a bare metal bolted connection. It could be a bare metal nameplate on a painted surface such as an oil filled circuit breaker or load tap changer. It could be a piece of electrical tape placed by the thermographer on bus bar to enable a decent reading. The list is long.

It turns out that for opaque objects, the emissivity and reflectivity are complementary. High emissivity means low reflectivity and vice versa. The conservation of energy law shows us that:

(1.1)

Greek letters for e, r, and t are typically used where emissivity is e, reflectivity is r, and transmissivity is t. For opaque targets, t = 0 and the equation reduces to:

(1.2)

Equation 1.2 is a powerful result. In simple terms it says that a high emissivity means a low reflectivity. A low reflectivity means a high emissivity. Thermographers like the emissivity to be as high as possible. They then get the most accurate reading as most of the radiosity is due to radiant energy emitted by the target. Modern IR cameras correct for emissivity with a modicum of user input. But the uncertainty in the measurement increases with decreasing emissivity. Our calculations show the measurement uncertainty gets unacceptably high for target emissivities below about 0.5.

Emissivity tables abound, but emissivity is a slippery slope. Above, we discussed emissivity as a material surface property. It is that, and more. The shape of an object affects its emissivity. For semi-transparent materials, the thickness will affect emissivity. Other factors affecting emissivity include: viewing angle, wavelength, and temperature. The wavelength dependence of emissivity means that different IR cameras can get different values for the same object. And they would both be correct! We recommend measuring the emissivity of your key targets under conditions they are likely to be monitored during routine surveys. A quality IR training course can teach you how. It is not difficult.

In general, dielectrics (electrically non-conducting materials) have relatively high emissivities, ranging from about 0.8 to 0.95. This includes well-painted metals. Unoxidized bare metals have emissivities below about 0.3 and should not be measured. Oxidized metals have emissivities ranging from about 0.5 to 0.9, and are the problematic category due to the large range of values. The degree of oxidation is a key ingredient to an object's emissivity. The higher the oxidation, the higher the emissivity.

For opaque objects, if you know the emissivity and the background (reflected) temperature, an IR camera with a temperature measurement feature can give temperatures accurate to within a few percent. To get temperature, the IR camera must extract just the fraction of the radiosity due to the energy emitted by the target. Fortunately, modern IR cameras are smart and can do this. They subtract the reflected component, then scale the result by the target emissivity. The resulting value can then be compared to a calibration table and temperature can be extracted.

What is a blackbody, a graybody, and a realbody?A blackbody is a perfect radiator. It has zero transmittance and zero reflectance. According to Kirchhoff's law, then, the emissivity of a blackbody is one. Blackbodies were first defined for visible light radiation. In visible light, something that doesn't reflect or transmit anything looks black. Hence the name. A graybody has an emissivity of less than one that is constant over wavelength. A realbody has an emissivity that varies with wavelength. IR cameras sense infrared radiant energy over a waveband. To get temperature, they compare the results explained above with a calibration table generated using blackbody sources. The implicit assumption is the target is a graybody. Most of the time this is true, or close enough to get meaningful results. For highly accurate measurements, the thermographer should understand the spectral (wavelength) nature of the target.

Max Planck is credited for developing the mathematical model for blackbody radiation curves. The accompanying figure shows the magnitude of emitted radiation due to an object's temperature vs. wavelength for various temperatures. Note the sun has a peak wavelength in the middle of our visible light spectrum.

Blackbody curves are nested. They do not cross each other. This means a blackbody at a higher temperature will emit more radiation at every wavelength than one at a lower temperature. As temperature increases, the wavelength span of radiation widens, and the peak of radiation shifts to shorter and shorter wavelengths. Note, the peak of infrared radiation at 300K (about 27C, 81F) is about 10 mm. Also, an object at 300K emits radiation only down to about 3 mm. Since our eyes are not sensitive beyond about 0.75 mm, we cannot see this. But if we warm the object up to about 300C, we can just begin to see it glow faintly red.

Why can't infrared film be used for thermal imaging applications?This is a question that people have been asking for more than twenty years. Understanding the Planck curves discussed above makes this an easy question to answer. An object must be hot enough to radiate at short enough wavelengths to expose infrared films that have special emulsions sensitive from 0.5 to 0.9 mm. Or, the object must reflect the radiation from a hot object. The latter has been the classic use of IR film. It was initially developed during World War II to detect camouflaged gun emplacements. The enemy did a good job of creating camouflage that looked like trees and bushes, which were difficult to detect when analyzing visible light aerial photographs. But healthy vegetation reflects sunlight in the near infrared quite strongly. Enemy camouflage did not. Infrared film was a real breakthrough and made the air photo interpreter's job much easier.

Infrared-sensitive photographic emulsions can be used to study the distribution of objects that are hot enough to emit infrared energy just below red heat levels such as stoves, engine parts, high-pressure boilers, etc. The range of temperatures that can be recorded is from approximately 250°C to 500°C (482°F to 932°F). In comparison, electronic thermography can be used on objects with temperatures ranging from -40 °C to more than 1500 °C (-40 °F to > 2730 °F). So if you wanted to see heat loss from your house with infrared film, it would have to be on fire! If you want to capture and image of your house in infrared under ambient, non-fire conditions, infrared film will not work. You must use a thermal infrared camera.

A more detailed discussion of infrared photography and thermal photography can be found at the Kodak website.

Where can infrared thermography be used?Infrared thermography is such a valuable and versatile tool that we cannot possibly list all the applications. New and innovative ways of using the technology are being developed everyday.

Thermography can be applied in any situation where a problem or condition can reveal itself by means of a thermal difference. For many situations, this is quite easy to apply; a thermal condition can be seen because the process involves release of thermal energy. An example of this is inspecting the condition of electrical distribution equipment. When electrical current passes through a resistive element, heat is generated. If the target emissivity is high enough, we can see that heat with an infrared camera. Sliding and bolted connections can become resistive through loosening, corrosion, etc. This increase in electrical resistance usually results in an increase in heat generation which the camera can quickly pick up. Sounds simple, and often it is. Frequently, it is not simple due to the nature of heat transfer. Good training is the key to successful application of infrared technology.

Another example is the inspection of concrete bridges. As many of us know, concrete can develop delaminations, which can lead to potholes. When a pothole develops, it is quite easy to detect; usually your tire and wheel "find" the hole and you end up with an unpleasant repair bill. Wouldn't it be great if we could find these before they cause problems? By cleverly using the sun's energy as a heating medium and viewing with an infrared camera, we find that subsurface delaminations have a different heating effect than the sound parts of the deck structure. The camera can see it. This example shows that even though the bridge deck doesn't generate heat it can still be analyzed with thermography given the proper conditions.

Let's consider another example of an application that takes advantage of passive heating or cooling. Recently developed composite aircraft materials are extremely sturdy and lightweight. These materials are vital to aircraft performance and airworthiness. However, the honeycomb structure of this material presents a potentially dangerous problem: water ingress.

It has been discovered that certain control surfaces tend to absorb water in the honeycomb structure, for reasons that are not fully understood. The problem is aggravated by the effects of lightning and hail, which cause barely visible impact damage. The water enters the honeycomb and freezes when the aircraft is at high altitude. As the ice expands, it breaks down the cells in the structure. This condition grows like a cancer and eventually jeopardizes the entire structural integrity of the component.

Until recently, the only effective method of diagnosing the problem was through radiography. While this is still the most accurate way, it has several disadvantages: it is expensive in time, equipment, and manpower, and can expose maintenance personnel to hazardous ionizing radiation.

Thermography can be an indispensable tool for inspecting planes for this problem. After the plane has landed, the ice remains at 0°C while it is melting. The rest of the plane has warmed to ambient temperatures on the approach. This provides an ideal opportunity to search for the ice pockets with a thermal imaging system while the plane is being serviced.